Will This Bridge Fall? It's Hard to Say

One lesson from the collapse of an interstate bridge in Washington state more than a week ago: When it comes to measuring a bridge's safety, so-called sufficiency ratings aren't sufficient.

After a truck crashed into an overhead girder on the steel bridge, sending cars and drivers into the Skagit River beneath it, news reports on the nonfatal accident fixated on the ratings inspectors give bridges, based on about 20 factors. But nearly half of any bridge's sufficiency rating, used until last year to determine which...